1. Field of the Invention
This invention relates generally to television receiver systems and, more particularly, to a method and apparatus for controlling the bandpass characteristic of the horizontal phase lock loop by altering the gain of the horizontal phase detector.
2. Description of the Prior Art
In most modern television receivers, a phase lock loop is employed to lock the horizontal oscillator in the horizontal sweep section of the receiver to the incoming separated horizontal synchronization pulses. This is done to achieve a high degree of noise immunity. In effect, the phase lock loop acts like a narrow bandpass filter centered on the horizontal frequency such that noise contamination in the separated synchronization pulse stream is rejected. Unfortunately, since the phase lock loop has a narrow bandwidth, it is characterized by a small pull-in range resulting in the possibility that pull-in (acquisition time) may be excessive. Pull-in or acquisition time refers to the time it takes the horizontal oscillator to lock onto the incoming synchronization pulse stream.
One solution to assuring proper pull-in over a full temperature range and at the same time provide a high degree of noise immunity is to employ a very stable horizontal oscillator. An alternative to this solution which enables the horizontal oscillator specification to be relaxed is to employ a phase lock loop which has two values of loop bandwidth. A first wide loop bandwidth may be used when the loop is out of lock with the synchronization pulse stream (i.e. during acquisition), and a second narrow loop bandwidth is employed when the horizontal oscillator is substantially locked onto the synchronization pulse stream. The first value yields good pull-in characteristics while the second permits the loop to have a high degree of noise immunity.
In most television receivers employing the above described switched bandwidth feature, a horizontal coincidence detector compares the phase of the horizontal synchronization pulse with that of the horizontal flyback pulse to determine if the loop is in a locked condition. That is, the output of the coincidence detector is a low voltage when the horizontal synchronization pulse is not coincident with the horizontal flyback pulse, and the output of the coincidence detector is high when the horizontal synchronization pulses and the horizontal flyback pulses are synchronized. With the aid of a comparator, the horizontal synchronization detector output voltage can be used to control the loop bandwidth in the desired manner by switching the phase detector gain.
With the trend towards a single integrated circuit chip television receiver containing a vertical coincidence detector, it is now possible to eliminate the horizontal coincidence detector since in the narrow bandwidth mode, the vertical coincidence detector gives an indication of horizontal as well as vertical lock. This would not only save semiconductor die area, but would also reduce the number of external components required.